1. Field of the Invention
The present invention relates to alignment marks and more particularly, to an alignment mark set comprising alignment marks that are formed in an exposure area and used to measure the alignment or overlay accuracy of patterns in the lithographic process for semiconductor device fabrication, and a method of measuring alignment accuracy of patterns using alignment mark sets.
2. Description of the Related Art
Generally, semiconductor devices comprise a lot of layers that form electronic elements, wiring lines, contacts, and so forth, which are stacked to each other along with interlayer dielectric layers. These stacked layers constitute an integrated circuit. Thus, to fabricate semiconductor devices, the stacked layers need to be patterned to form desired electronic elements, wiring lines, and so forth using well-known lithography and etching techniques.
With the lithography and etching techniques, it is very important not only to transfer a desired minute pattern to a specific layer on or over a semiconductor wafer as closely as possible but also to overlay an upper pattern to a lower one as correctly as possible. Thus, in the lithography process, the pattern of a resist layer, which is formed on a layer to be etched or patterned (i.e., a target layer), needs to be aligned on a desired lower pattern located below the target layer with high accuracy. In particular, circuits and elements provided in a recent semiconductor device have been increasingly miniaturized and therefore, the need to raise the overlay accuracy (i.e., alignment accuracy) of patterns has been becoming stronger.
Conventionally, to meet the above-described need, xe2x80x9calignment marksxe2x80x9d have been usually formed along with a pattern for desired circuits and/or elements, thereby measuring the alignment accuracy using the alignment marks.
FIGS. 1 and 2 show an example of the conventional alignment marks of this sort, which is formed on a semiconductor substrate or wafer.
The conventional alignment mark 100 shown in FIGS. 1 and 2, which has been usually used for this purpose, comprises two mark elements 118 and 119. The inner element 119 is located in the outer element 118. Each of the elements 118 and 119 is square in plan shape. The mark 100 is formed in the following way.
First, as shown in FIG. 2, a first layer 122 is formed on the surface of a semiconductor substrate or wafer 121 and then, a patterned resist layer (not shown) is formed on the first layer 122. The patterned resist layer is formed by the lithography technique. Then, using the patterned resist layer as a mask, the first layer 122 is selectively etched, thereby forming a first or lower circuit pattern (not shown) and the outer square element 118 of the alignment mark 100 in the layer 122. As clearly seen from FIG. 2, the element 118 is a square hole or opening of the layer 122.
Subsequently, a second layer 123 is formed on the first layer 122 thus patterned so as to contact the bottom and side faces of the outer element 118 (i.e., the hole of the first layer 122). Then, a resist layer (not shown) is formed on the second layer 123 thus formed, and is patterned by the lithography technique, thereby forming a second or upper circuit pattern (not shown) and the inner square element 119 of the alignment mark 100 on the second layer 123 in the hole 118 of the first layer 122.
As clearly seen from FIG. 2, the inner element 119 of the mark 100 is a square part of the resist layer and located in the hole or outer element 118. The patterned resist layer thus formed is used as a mask in the next etching process for patterning the underlying second layer 123.
The alignment mark 110, shown in FIG. 3, comprising the outer and inner elements 118 and 119 thus formed is used to measure the alignment accuracy between the first circuit pattern formed by the first layer 122 and the second circuit pattern of the resist layer. In this case, the relative positional relationship between the outer and inner elements 118 and 119 is measured.
For example, as shown in FIG. 2, the distance d1 between the inner side face 118a of the outer element 118 and the facing side face 119a of the inner element 119 is measured. At the same time as this, the distance d2 between the opposite inner side face 118b of the outer element 118 and the facing side face 119b of the inner element 119 is measured. If the values of the distances d1 and d2 are equal, it is judged that the first or lower circuit pattern is overlaid with the second or upper circuit pattern correctly (i.e., with desired alignment accuracy). Sometimes, it is judged whether or not the value of the difference (d1xe2x88x92d2) between the distances d1 and d2 is within a specific range. In any of these cases, no subsequent process is applied unless it is judged that the first or lower circuit pattern is overlaid with the second or upper circuit pattern with desired alignment accuracy.
FIG. 3 shows the layout or arrangement of the conventional alignment marks in an exposure area on a semiconductor wafer. As seen from FIG. 3, four alignment marks 110, 111, 112, and 113 are provided in one of rectangular one-shot exposure areas 107 arranged on a semiconductor wafer 10. Each of the marks 110, 111, 112, and 113 has the same structure as the conventional mark 100 shown in FIGS. 1 and 2. Specifically, each of the marks 110, 111, 112, and 113 comprises the square outer and inner mark elements 118 and 119. The set of the four alignment marks 110, 111, 112, and 113 may be termed the xe2x80x9cconventional alignment mark setxe2x80x9d.
In the rectangular exposure area 107 in FIG. 3, the two marks 110 and 112 are located on the longitudinal, central axis 115 of the area 107, where the direction along the axis 115 is defined as the Y direction. Since the marks 110 and 112 are on the axis 115, they are positioned at the middle of the short sides 107a and 107c of the area 107. The mark 110 is close to the upper short side 107a while the mark 112 is close to the lower short side 107c. The other marks 111 and 113 are located on the lateral, central axis 114 of the area 107, where the direction along the axis 114 is defined as the x direction. Since the marks 111 and 113 are on the axis 114, they are positioned at the middle of the long sides 107b and 107d of the area 107. The mark 111 is close to the right long side 107b while the mark 113 is close to the left long side 107d. A desired circuit or element pattern (not shown) is typically located among the four marks 110, 111, 112, and 113 in the area 107.
To measure the alignment accuracy along the X direction, the alignment marks 111 and 113 located on the lateral axis 114 are used. Specifically, the distances d1 and d2 between the outer and inner elements 118 and 119 along the X direction is measured for each of the marks 111 and 113. Then, the difference (d1xe2x88x92d2) of the distances d1 and d2 is calculated. Thus, the alignment accuracy along the X direction is determined by the value of the difference (d1xe2x88x92d2) thus calculated.
Similarly, the alignment accuracy along the Y direction is measured using the alignment marks 110 and 112 located on the longitudinal axis 115. Specifically, the distances d3 and d4 between the outer and inner elements 118 and 119 along the Y direction is measured for each of the marks 110 and 112. Then, the difference (d3xe2x88x92d4) of the distances d3 and d4 is calculated. Thus, the alignment accuracy along the Y direction is determined by the value of the difference (d3xe2x88x92d4) thus calculated.
Actually, a lot of the rectangular exposure areas 107 shown in FIG. 3, each of which includes the conventional alignment mark set comprising the four marks 110, 111, 112, and 113, are regularly arranged on the semiconductor wafer 121, as shown in FIG. 4. In FIG. 4, the exposure areas 107 are arranged in a matrix array on the wafer 121. Needless to say, the alignment accuracy is measured in each of the areas 107 using the marks 110, 111, 112, and 113.
With the conventional alignment mark set comprising the four marks 110, 111, 112, and 113, some of the marks 110, 111, 112, and 113 are located close to each other. For example, as shown in FIG. 4, the mark 111 in one of the areas 107 and the mark 113 in another of the areas 107, which are surrounded by an ellipse 116, are close to each other. Also, the mark 112 in one of the areas 107 and the mark 110 in another of the areas 107, which are surrounded by an ellipse 117, are close to each other. In this case, there arises a problem that the inner elements 119 of the marks 110, 111, 112, and 113 do not have desired contours or edges, which is explained in detail below with reference to FIG. 5.
For example, with the inner element 119 of the alignment mark 111 surrounded by the ellipse 116, the outer side face 119b tends to be tilted, as shown in FIG. 5. The reason why the outer side face 119b is made oblique is not known clearly; however, it is thought in the following way.
Specifically, it is assumed that the exposing light irradiated to the resist layer for the inner element 119 of the mark 111 is affected by the next mark 113 to the mark 111. Thus, the resist layer is not exposed to the light as desired. As a result, the top corner of the side face 119 is broken or deformed in the development process of the resist layer.
If the element 119 has the oblique side face 119b shown in FIG. 5, the contour or outline of the side face 119b is unable to be detected or observed correctly. Thus, the distance d2 between the side face 119b and the opposing inner face 118b of the outer element 118 tends to be measured d2xe2x80x2. As a consequence, the alignment accuracy thus observed tends to include some divergence or error of [(d1xe2x88x92d2xe2x80x2)/2], which degrades the measurement accuracy.
Moreover, there is a possibility that the inner element 119 of the mark 110, 111, 112, or 113 is deformed due to applied etching action and/or applied heat in a subsequent process or processes. In this case, because of synergism of the deformation and the tilting/breaking of the side face 119b, the measurement accuracy deteriorates more.
Accordingly, an object of the present invention is to provide an alignment mark set that facilitates the formation of a desired contour of each alignment mark, and a method of measuring alignment of patterns using the set.
Another object of the present invention is to provide an alignment mark set that suppresses the degradation of measurement accuracy for alignment of patterns, and a method of measuring alignment of patterns using the set.
Still another object of the present invention is to provide an alignment mark set that ensures high measurement accuracy, and a method of measuring alignment of patterns using the set.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, an alignment mark set is provided. The set comprises:
(a) a first alignment mark formed in an exposure area;
the area having a periphery, first central axis, and
a second central axis perpendicular to the first axis;
the first alignment mark being located near the first central axis and apart from the second axis;
(b) a second alignment mark formed in the exposure area;
the second alignment mark being located near the second central axis and apart from the first axis; and
(c) when the exposure areas are regularly arranged in such a way as to have a same orientation in a plane, each of the first and second alignment marks in one of the sets is not located close to the first and second alignment marks in another of the sets, thereby ensuring irradiation of exposing light to all the areas.
With the alignment mark set according to the first aspect of the present invention, the first alignment mark is formed in the exposure area in such a way as to be located near the first axis of the area and apart from the second axis thereof. On the other hand, the second alignment mark is formed in the exposure area in such a way as to be located near the second axis of the area and apart from the first axis thereof.
Thus, when the alignment mark sets according to the first aspect are arranged to be adjacent to each other in a plane on use, each of the first and second alignment marks in one of the set is not located close to the first and second alignment marks in another of the sets. Accordingly, even if exposing light is irradiated to all the exposure areas thus arranged, none of the first and second alignment marks in one of the sets are affected by the first and second marks in another of the sets.
As a result, the formation of a desired contour of each of the first and second marks is facilitated, which suppresses the degradation of measurement accuracy for alignment of patterns. This means that high measurement accuracy is ensured.
It is preferred that each of the first and second marks is square.
In a preferred embodiment of the alignment mark set according to the first aspect, there are additionally provided with;
(c) a third alignment mark formed in the exposure area;
the third alignment mark being located near the first central axis and apart from the second axis;
the third alignment mark being shifted from the first central axis along the second central axis in an opposite direction to the first alignment mark; and
(d) a fourth alignment mark formed in the exposure area;
the fourth alignment mark being located near the second central axis and apart from the first axis;
the fourth alignment mark being shifted from the second central axis along the first central axis in an opposite direction to the second alignment mark.
In another preferred embodiment of the alignment mark set according to the first aspect, there are additionally provided with;
(c) a third alignment mark formed in the exposure area;
the third alignment mark being located near the first central axis and apart from the second axis;
the third alignment mark being located on the first central axis along with the first alignment mark; and
(d) a fourth alignment mark formed in the exposure area;
the fourth alignment mark being located near the second central axis and apart from the first axis;
the fourth alignment mark being located on the second central axis along with the second alignment mark.
In this embodiment, it is preferred that the first and third marks are apart from the periphery on opposite sides at equal distances while the second and fourth marks are apart from the periphery on opposite sides at equal distances. More preferably, the first and third marks are apart from the periphery at equal distances of 100 xcexcm or greater while the second and fourth marks are apart from the periphery at equal distances of 100 xcexcm or greater.
According to a second aspect of the present invention, a method of measuring alignment of patterns is provided, where alignment mark sets are regularly arranged in such a way as to have a same orientation in a plane on use. This method comprises the steps of:
(a) arranging the alignment mark sets on the plane so as to be adjacent to each other;
each of the sets comprising
(a-1) a first alignment mark formed in an exposure area;
the area having a periphery, first central axis, and a second central axis perpendicular to the first axis;
the first alignment mark being located on the first axis near the periphery;
(a-2) a second alignment mark formed in the exposure area;
the second alignment mark being located on the second axis near the periphery;
(a-3) a third alignment mark formed in the exposure area;
the third alignment mark being located on the first axis near the periphery on an opposite side to the first mark;
(a-4) a fourth alignment mark formed in the exposure area;
the fourth alignment mark being located on the second axis near the periphery on an opposite side to the second mark; and
(b) measuring alignment accuracy using the first mark in a first one of the sets and the third mark in a second one of the sets adjacent to the first one of the sets and the second mark in one of the first one of the sets and the fourth mark in the second one of the sets;
wherein unopposed sides of the first and third marks located on opposite sides to each other with respect to the periphery of the area and unopposed sides of the second and fourth marks located on opposite sides to each other with respect to the periphery of the area are used for measuring.
With the method according to the second aspect of the invention, unopposed sides of the first and third marks located on opposite sides to each other with respect to the periphery of the area and unopposed sides of the second and fourth marks located on opposite sides to each other with respect to the periphery of the area are used for measuring. Thus, even if exposing light is irradiated to all the exposure areas thus arranged, none of the first and second alignment marks in one of the sets are affected by the first and second marks in another of the sets.
As a result, the formation of a desired contour of each of the first and second marks is facilitated, which suppresses the degradation of measurement accuracy for alignment of patterns. This means that high measurement accuracy is ensured.
It is preferred that each of the first and second marks is square.